Arterial graft prosthesis

ABSTRACT

An arterial graft prosthesis comprises a first interior zone of a solid, segmented polyether-polyurethane material surrounded by a second zone of a porous, segmented, polyether-polyurethane and a third zone immediately surrounding said second zone and of a solid, segmented polyether-polyurethane. The interior zone may have a lining or blood interface of a microporous zone of segmented polyether-polyurethane, and the exterior, third zone may be surrounded by a tissue interface of a microporous zone of segmented polyether-polyurethane. In some instances the exterior may be confined by a tube of substantially non-stretchable netting fastened in place at chosen, spaced intervals or other forms of reinforcement may be employed. Other materials can be used.

BACKGROUND OF THE INVENTION

Prosthetic arterial grafts have been available to the medical professionfor thirty years or more. However, during that thirty years thedevelopment of such grafts have been limited to those formed of textilefabrics and of semi-rigid plastics such as Teflon which have been madesomewhat flexible by distension or stretching so that microscopic poresare produced which, although too small to permit the passage of blood,do permit some degree of flexibility. Such porosity does allow suchgrafts to eventually leak under certain conditions. Textile arterialgrafts are generally a single tubular structure. Arterial graftprostheses of stretched, semi-rigid plastics have been made of multipleparts or tubes but the resultant structure has not been homogeneous orattached so that it may be separable under normal conditions of use suchas during the suturing of the graft in place.

In the development of an arterial graft prosthesis it must be recognizedthat the optimal prosthesis should have static and dynamic elasticmoduli and pressure distension in both the radial and axial directionswhich closely match those for normal human arteries of the samediameter. Moreover, the wall thickness should be very close to that ofthe human artery and should resist kinking when bent as well as donatural arteries. The prosthesis should have uniform homogeneousphysical properties fully along its length so that the surgeon may cutany length he desires. Moreover, it should be easily sutured with thesame needle penetration force and suture pull through force as isrequired with natural arteries. The suture should not pull out nor tearto any greater extent nor with any less force than with the naturalartery. The prosthesis should be impervious to blood not only along themajor portion of its length but also where the customary needle holesare placed during suturing. This, particularly, has not been possiblewith graft prosthesis of the prior art in which leaking at suture pointsusually exists until a thrombus is formed. Moreover, the prosthesisshould inhibit tissue growth throughout the graft structure which wouldresult in the stiffening of the graft itself. It should be compatiblewith blood and tissue and should also provide attachment to externaltissues for fixation and avoidance of trapped fluids inside a loosetissue capsule. The graft should remain patent and unobstructedindefinitely without any inherent clot or generation of thrombo-emboli.These objectives have not been met by the arterial graft prosthesis ofthe prior art.

BRIEF SUMMARY OF THE INVENTION

A representative arterial graft prosthesis in accordance with theinvention comprises at least two concentric zones of elastomer materialhomogeneously joined together to form a single tube with the elastomerin one of the zones being porous. In other embodiments such a tube is,in turn, surrounded by a third zone of a solid, segmented elastomer.Also, the interior surface or the exterior surface, or both, of thementioned three-zone tube may also be augmented from time to time andfor different conditions.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a longitudinal, diametrical cross section of a compositearterial graft showing not only the three mentioned zones of segmentedelastomer but also an interior and exterior lining zone and an exteriorconfining reinforcement.

FIG. 2 is a view comparable to FIG. 1 but in which the confiningreinforcement, instead of being external and of woven material, isinternal and of a helically wrapped thread.

FIG. 3 is a view comparable to FIG. 1 but without reinforcement.

FIG. 4 is another view comparable to FIG. 2 but with some smallvariations.

FIG. 5 is a longitudinal, diametrical cross-section showing a preferredvariation having reinforcement in the porous zone.

FIG. 6 shows a preferred embodiment particularly adapted to smalldiameter grafts.

FIG. 7 is a view comparable to FIG. 1 but having a corrugated internalconstruction.

FIG. 8 is a view like FIG. 7 but in which the entire construction iscorrugated.

FIG. 9 is a view like FIG. 7 but in which the exterior is crrugated,while the interior is smooth.

FIG. 10 is another variation showing reinforcement in the form of hoopswhich are helical and augmented.

FIG. 11 is an end view of the structure of FIG. 10 with a portion brokenaway to show the interior construction.

FIG. 12 is an isometric view showing an embodiment with hoopreinforcements.

FIG. 13 is a view like FIG. 12 with different-shaped hoops.

FIG. 14 is a view like FIG. 1 with a plurality of internal concentrichelical reinforcements.

FIG. 15 is a view like FIG. 14 in which the reinforcements areconcentric hoops rather than helical.

FIG. 16 is like the FIG. 14 version, but the reinforcement is by asingle row of rings rather than by a pair of coaxial helices.

FIG. 17 is like the FIG. 16 arrangement but the reinforcing rings are ofa flat stock rather than round stock.

DETAILED DESCRIPTION

While the drawings herein show the various zones of the arterial graftseparated by solid lines, it must be recognized that the zones arehomogeneously attached to each other while the substance of at least oneof the zones is in a liquid or semi-liquid state. The various zones areformed of the same general material such as polyurethane. Thepolyurethane is dissolved in a solvent and applied as a viscous liquid.The solvent within the liquid penetrates the surfaces of the attachedzones and provides for homogeneous mixing of the polymers and adhesionas if it were of one material. Consequently, the interface between zoneshas some finite dimension of thickness not shown in the drawing andhaving a composition which is a blend of the two adjacent zones.

In the representative version of the arterial graft shown in FIG. 1,there is provided, symmetrical with a center line 5, an inside tubularzone 6. This is a generally impervious zone of a segmentedpolyether-polyurethane material which is continuous and is largely abarrier to the various liquid materials with which it is normallyassociated. The zone 6 is approximately one to six mils in radialthickness and is generally a radial bar to the blood which flows throughthe graft under normal pressures and actions.

Directly surrounding the tubular zone 6 there is an intermediate zone 7also fundamentally of polyether-polyurethane but preferably of somewhatgreater thickness--generally from 10 to 80 mils. The zone 7 is of aporous nature, being from thirty percent to ninety percent void or openvolume. The void volume may be of uniformly sized and distributed poreswith the pores 8 ranging from one to one hundred fifty microns in size.

Usually, the intermediate zone 7 is directly surrounded by anencompassing or outward zone 9. This also is of a segmentedpolyether-polyurethane material corresponding to that in the zone 6 andhaving a lack of pores or orifices, being continuous like the zone 6 andso distinguished from the porous zone 7. The radial dimension of thezone 9 is from one to six mils. The portions of this form of substituteblood vessel thus far described are all made of segmentedpolyether-polyurethane zones of different sizes and with the centralzone, only, containing a number of cells or spaces or voids.

Reference herein is made primarily to the zones being of a segmentedpolyether-polyurethane material, and that has proved to be mostsatisfactory in practice. It must be noted, however, that of thenumerous elastomers available (for example, silicone rubber) in variousinstances there may be used elastomers that are not segmented and arenot polyesters nor polyethers. Much of the actual work herein has beendone with a segmented polyether-polyurethane material, specifically suchmaterial sold under the trademark "Biomer", and so, for convenience,those materials are referred to herein. Yet, it must be recognized thatcomparable and substitute materials may be used or may become available.

Construction of an arterial graft prosthesis, as defined herein, resultsin static and dynamic moduli and pressure distortions in both radial andaxial directions closely to match such distensions of normal humanarteries of comparable diameter. Consequently, even after clamping, theprosthesis of the present invention, unlike the prior art prostheses,recover in the same manner as do natural arteries. Also, the wallthickness used is very close to that of human arteries of a comparablediameter. Furthermore, these prostheses, when formed, resist kinkingwhen bent as well as the normal, natural arteries. There are uniform,homogeneous physical properties along the entire length of the insert orgraft, so that the surgeon can cut an insert to any length he desireswith uniform results.

The prostheses defined suture as easily and with substantially the sameneedle penetration force and suture pull-through force as in the case ofnatural arteries. Also, sutures made in the prostheses do not pull outor tear to any greater extent or with any less force than those in anatural artery. The grafts defined, when formed, are impervious or leaktight to the circulating blood. The customary needle holes aresubstantially immediately self-closing, so that they do not leak anycontained blood. Furthermore, the prostheses inhibit tissue growththroughout the graft structure and so prevent a resultant stffening ofthe graft. They readily provide appropriate fixation for external tissueattachment and ready avoidance of trapped fluids inside the tissuecapsule. The prostheses are quite compatible with the customary bloodand adjacent tissues. Also, the formed, indicated prostheses generallyremain open and unobstructed indefinitely and without any adherent clotor the generation of internal thromboemboli. The net result is that thedescribed prostheses afford an arterial replacement which is in mostrespects virtually indistinguishable from the original native artery.

In addition to the zones 6, 7, and 9, the embodiment of FIG. 1 alsoincludes a microporous blood interface 10. In an arterial graft ofapproximately six millimeter internal diameter or more, the bloodinterface 10 may comprise a zone of "Biomer", having interior pores of adiameter and depth ranging from five to one hundred microns,approximately. Preferably, the microporous tissue is treated to behydrophilic or hydrophobic, for instance, by subjection to known gasplasma or electrical discharge methods. The so-treated microporoustissue interface is effective as an anchoring substrate for a developingpseudointima which forms blood constituents. This pseudointima is atissue layer which must adhere to the blood interface and remains quitethin. If desired, the hydrophilic or hydrophobic microporous bloodinterface may be coated with an antithrombin such as albumin, gelatin,glycoproteins, bonded heparin or comparable material to prevent or todiminish any early thrombus formation. Such initial coatings, in use,may gradually be replaced with the developing pseudointima as described.

About the outside of the graft is an adherant surrounding tissueinterface 11. The material of the interface 11, like the blood interface10, may also be a microporous "Biomer" but with slightly largerpores--in the range of thirty to one hundred fifty microns. The surfaceof this interface may also and similarly be made hydrophilic orhydrophobic.

Particularly for the porous core 7 but also for the interfaces 10 and11, the homogeneous pores are initially formed by the use ofparticulates such as salt (NaCl) or sodium bicarbonate, which isultimately largely removed by diffusion in a water or very dilute acidbath. The sodium bicarbonate also acts as a blowing agent in that theCO₂ is released and thus decreases the amount of salt to be removed fromthe core. The particulates utilized for this purpose are screened toafford a very narrow range of sizes, so that the pore sizes themselvesare confined to a very narrow range. The result is a porous or foam-likestructure containing closed-cell or open-cell voids with a substantiallyreduced general density.

The salt particles, for example, and the "Biomer" or core solution arecompletely and homogeneously mixed to form a slurry. Different slurries,with or without salt particles, are then used to form the various zoneson a mandrel. The first slurry (with salt or sodium bicarbonate particlesize to produce pores of from five to one hundred microns) is applieddirectly to the mandrel by dipping, coating or doctoring to form thezone 10. This is followed by an unsalted solution to form the zone 6 andthe sequence is continual until the entire graft is fabricated. Thecoatings on the mandrel are then thoroughly dried to remove the solventand then the salt or bicarbonate particles are removed in a water bathat about 60° C. There are water-filled voids so created by thedissolution and diffusion of the salt particles. The particle size andconcentration of particles are arranged to control the density orporosity and pore size. For example, sodium chloride particles of aboutfifty micron average size are used for the zone 7 and the optimum rangeis within about one to one hundred fifty microns. The total void volumeis about fifty percent of the total layer volume, the range being fromabout thirty percent to ninety percent.

The result of the foregoing is the production of a readily patent andclinically superior tubular graft which simulates very closely theproperties of a natural artery. This provides that the artificial graftcan be sutured to adjacent arteries very much as though a natural arterywere utilized. The dimensional and distensional simulation by theartificial material to the natural material reduces or eliminatessutureline discontinuities and obstructions. Furthermore, grafts of thepresent prostheses behave very much like the natural artery, so that thesurgeon's skill and experience are fully utilized. Also, the grafts ofthe present prostheses provide the same suturability, freedom fromkinking, clamping characteristics, impermeability, biocompatability,antithrombogenicity, patency, and other advantages of natural arterialmaterial.

Although the present artificial construction has two or often threezones, nevertheless the several zones allow for the provision ofindividual or separate component characteristics, preferably all basedon the polyether-polyurethane structure.

In some instances, it is desirable to surround the exterior of theartificial artery so provided with a confining netting 12 of "Dacron" orthe like or a circumferential winding of a suitable filament 13 (seeFIG. 2) which may be formed of "Dacron", solid elastomer, wire or thelike. When external, as in FIG. 1, this reinforcement is customarilyadhesively affixed at spaced intervals and loosely surrouds theremainder of the artificial artery. The netting comes into playprimarily only in the event there is a substantial expansion of theartery. The netting confines the amount of such expansion to precludeundue stretching and thinning of the artery walls.

As an alternative to the attachment of the reinforcing material to theouter zone of the prosthesis, as shown in FIG. 1, it may be formedwithin one or more of the zones of elastomer as particularly shown inFIG. 2, the purpose here also being to preclude undue expansion of theartificial artery.

Clinical experience has indicated that an artificial artery constructedas described herein and particularly fabricated primarily ofpolyether-polyurethane is virtually indistinguishable from the naturallyoccurring artery which it replaces and affords a long-term, effectiveand trouble free substitute for the originally occurring, naturalartery.

The alternative embodiment of FIG. 2 differs from that of FIG. 1 notonly in the type of reinforcing material, but also in that the tissueinterface 11 is eliminated.

Another modified graft as shown in FIG. 3 is a section 21 acting as anartery generally symmetrical about a longitudinal axis 22 andparticularly inclusive of an inside zone 23 of relatively solidpolyether-polyurethane, preferably of segmented polyether-polyurethane.The zone 23 is arranged symmetrically about the axis 22 or approximatelyso and on its inner surface is covered particularly with an inner coat24 of a micro porous blood interface of polyether-polyurethane. Thepores in the interface coat 24 are in the range of five to one hundredmicrons in size and depth, and the coat itself is treated primarily tobe hydrophilic.

The inside zone 23 and the inside coat 24 line a tubular zone or body 26of a porous, segmented polyether-polyurethane having pores of about oneto one hundred fifty microns in size. The pores are sufficient in numberand disposition to allow from about thirty percent to ninety percent ofthe zone 26 to consist of pores.

Around the tubular body 26 there is a further zone 27 of relativelysolid polyether-polyurethane about one to six mils thick. Finally,surrounding the zone 27 there is a generally exposed microporous tissueinterface 28 comprised of polyether-polyurethane having pores of aboutthirty to one hundred fifty microns in size and depth. This exteriorinterface 28 likewise is treated to be hydrophilic.

It is found that with these materials and this general arrangement andconstruction, many of the objects of the invention are attained in anacceptable fashion. The size characteristics of the structure are wellfixed and remain stable over a very long time. The wall thickness isclose to that of natural human arteries of similar duties and diameters,and the materials resist kinking when bent around short-radius curves atleast as well as natural arteries do. The material is uniform throughoutits length, so that a fabricated tube can be cut for use of any selectedportion. The material sutures easily and with similar needle techniquesto those used with natural arteries. The material does not rip nor tearany more easily than natural arteries do. Further, the materialsutilized provide a wall which is virtually impervious or leak-tight toblood. Tissue does not tend to grow into or stiffen the material afterinstallation.

It is therefore quite possible by utilizing the lay-up shown in FIG. 3and utilizing the materials specified in connection therewith and of thenature, size and characteristics stated to provide an excellent,long-term, readily handled and effective substitute for naturalarteries.

The arrangement of FIG. 4 is very similar to that of FIG. 2. It differsin that the microporous blood interface is eliminated. In the embodimentof FIG. 4, the impervious inner zone 28 may be formed ofpolyether-polyurethane having an ultrasmooth surface treated by gasplasma methods, for instance, to obtain an optimal hydrophobic surface.With such a construction, the embodiment of FIG. 4 is particularlysuited to grafts of 5 mm internal diameter and smaller. Such smallergrafts may not be able to sustain a pseudointima without substantialrisk of occlusion and the smooth hydrophobic surface will serve toprevent thrombus formation. Such antithrombogenic surfaces may remain"clean" except for a thin glycoprotein layer.

The embodiment of FIG. 5, a preferred embodiment, is very similar tothat of FIG. 1. The latter embodiment differs from the former in that aspiral reinforcement 29 is disposed in the intermediate porous zone 7rather than a net reinforcement about the outer porous zone 11.Moreover, in the embodiment of FIG. 5, the impervious zone 9 iseliminated. While the embodiment of FIG. 5 includes a microporous bloodinterface 10, it should be understood that a hydrophobic copolymer couldbe added, particularly on small grafts. The spiral reinforcement 29improves the anti-kinking characteristics of the graft; achieves anadequate radial elastic modulus and at the same time avoids any sharp orspiny protrusions when the graft is cut through. To this end it has beenfound that the tensile elastic modulus of the spiral reinforcementfilament itself should be in the range of from 10,000 to 2,000,000 psi.Moreover, the ratio of the distance between spiral loops (the pitch ofthe spiral) to the diameter of the filament itself should be in therange of from 1.5 to 5.

The embodiment of FIG. 6 is particularly suited for small grafts. Inthis instance the structure closely resembles that of FIG. 5 but theblood interface 10 is eliminated. With such a structure the innerimpervious zone 28, comparable to the zone 6 in FIG. 5, may be treatedto make its surface hydrophobic and thus blood compatible. Graftsconstructed in accordance with this embodiment may have an internaldiameter as small as one millimeter.

There may be another alternate structure of the tubular substituteartery provided, as shown in FIG. 7. In this instance the interior issymmetrical about an axis or center line 41 as before. The axis isgenerally symmetrically surrounded by a circular, thin solid elastomerzone 42 of segmented polyether-polyurethane. In this instance the zone42 may be clear or lined--if lined, then having another solid elastomerzone 43 on the interior thereof. Surrounding the zone 42 there is arelatively thick, generally porous, annular zone 44. This, in turn, issurrounded by an external zone 46 of a thin solid elastomeric materialin turn encased in a porous elastomer zone 47. A unique feature in thisinstance is that the major generally porous zone 44 is especiallyaugmented by an enclosed, longitudinally extending, solid or highdensity porous elastomer 48 formed with corrugations 49, thecorrugations being either parallel and circular or contoured in a spiralpath.

While it is usually customary to provide each of the vessels as asymmetrical construction of relatively unlimited length and extendingalong a central axis 51, the configuration need not include an entirelycylindrical enclosure as shown in FIG. 8. For example, and especiallyfor use in relatively large diameter grafts; i.e. over eight millimetersinside diameter, there can be provided a zone setup very much aspreviously described but with the zones configured around the axis 51 ina convoluted structure 52. That is, the interior surface need not begenerally smooth, but may be undulatory or corrugated, with thedifferent undulations either having parallel circumferential paths orjoined in a spiral path. In this instance, as before, the inner bloodcontacting surface 53 may be a microporous blood interface. Nextadjacent is a solid elastomer zone 54, while surrounding that is aporous elastomer zone 56 of medium density. Around that next to theoutside there is an impervious zone 57, and finally a porous elastomertissue interface or zone 58 on top of everything. The embodiment of FIG.8, then, is similar to that of FIG. 1 except that the reinforcement 12of FIG. 1 is replaced by the corrugated configuration of the overallprosthesis. It should be recognized that the reinforcement of otherembodiments described above, such for example, as that of FIG. 5, mayalso be replaced by the corrugated configuration as shown in FIG. 8.Such configuration are particularly suitable for grafts having aninternal diameter of from about 10 to 30 millimeters.

In another variation, as particularly shown in FIG. 9, there is a vesselsymmetrical about a central axis 61. The inside zone 62 is formed of aporous elastomer. Encasing this is a solid elastomer zone 63 which, inturn, is surrounded by a porous elastomer zone 64. The zone 64 isencased by a solid elastomer, undulatory zone 66 itself coated orsurrounded by an external zone 67 of a microporous tissue interface.Alternatively, the inner zone 62 may be eliminated and the smoothsurfaced impervious zone 63 treated to be blood compatible.

A somewhat different approach is shown in FIGS. 10 and 11. An encasingwall shown generally at 81 is of a suitable material and multiple zonesas previously described. Embedded in the wall and symmetrical about thecentral axis 82 is a helical reinforcement 83. This is preferablyfabricated of a solid or quite dense porous elastomeric material. Thereinforcement 83 in turn can also be wrapped with a helical filament 84comprised of plastic thread or metal wire.

As shown particularly in FIG. 12, there is arranged around an axis 91,as before, first an inner surface zone 92, followed by an imperviouszone 93, a relatively porous, thick zone 94, a relatively solid outerzone 96 and finally an outside, thin, elastomeric zone 97. Particularly,the zone 94 is especially characterized by a number of axiallyseparated, embedded rings 98 to afford hoop strength and to maintain theaxial disposition of the materials. The hoops 98 can be of a solidelastomer; of a high density, porous elastomer; or of a rigid plasticsuch as a polyester. They even can be of metal such as stainless steelwire.

A variation on this theme is shown generally in FIG. 13. The centralaxis 101 is as before and marks the center line of an inner microporouszone 105 within a solid zone 106 in turn within a porous zone 107surrounded by an outer solid zone 108 and an encompassing tissueinterface 109. In this instance, there are inclined hoops 110. Insteadof being circular in axial cross-section as the hoops 98 of FIG. 12, thehoops 110 are rather of a radially elongated cross section approximatelyelliptical in pattern. The artery so furnished is relatively strong in aradial direction or against radial pressure.

In a similar arrangement in FIG. 14, around the central axis 111 issubstantially the same arrangement of zones 112, 113, 114, 116 and 117.In addition, there is an inner helical body 118 of wire, plastic orelastomer as well as a surrounding, outer helical body 119. Because ofthe different diameters of the helical bodies, the pitches of theirindividual convolutions vary somewhat. The general homogeneity of theblood vessel wall is not adversely affected by the periodical appearanceof the reinforcements.

As a variation on this theme, there is provided, as shown in FIG. 15, acomparable arrangement in which the various zones 121, 122, 124, 126 and127 are symmetrical about a through axis 123. In this instance, thereinforcements are again in the generally porous central zone 124 andcomprise wires 128 and 129, of either metal, elastomer or plasticdisposed near the center and outside respectively. These wires formnested, circular rings. There is no axial transmission of forceslongitudinally along the length of the FIG. 15 tube by the separaterings 128 and 129, as there may be along the convolutions 118 and 119 inthe FIG. 14 version.

In the FIG. 16 version, the zones 132, 134, 136, 137 and 138 aredisposed about an axis 133. Symmetrical with the axis 133 arereinforcing rings 139. This arrangement is not especially restrictedlongitudinally, but is restricted circumferentially.

In the arrangement of FIG. 17 the axis 141 is encompassed by the variouszones 142, 143, 144, 146 and 147. Within the zone 144 and around theaxis 141 are spaced rings 148 of flat wire or plastic stock. Again,these are not constrictive in an axial direction, but, even more thanthe FIG. 16 version, afford substantial radial restriction.

With all of these arrangements, it is found that the objects of theinvention are in general met, and that the natural arterial constructioncan be replaced by any of the constructions shown herein keeping in mindthat the various forms of reinforcement may be utilized not only in themore complex joined zones of material, and any of the reinforcementsdescribed could be included in such a simple structure. Due regard maybe had to the relative dimensions in diameter and length involved. Takeninto appropriate account should be the juxtaposition of the variousmaterials and their own individual and relative dimensions. It has beenfound that substantially impervious, long-lived blood carrying vesselseffective under normal human pressures and conditions are well providedin each instance.

I claim:
 1. An arterial graft prosthesis formed of a core zone of porouselastomer disposed about the longitudinal axis of the prosthesis, aninner zone of solid elastomer concentric with and homogeneously joinedto the inside of said core zone, an outer zone of porous elastomer, saidouter zone of porous elastomer being concentric with and homogeneouslyjoined to the outside of said core zone.
 2. An arterial graft prosthesisas defined in claim 1, together with a blood interface zone ofmicroporous elastomer concentric with and homogeneously joined to theinside of said inner zone of solid elastomer.
 3. An arterial graftprosthesis as defined in claim 2 wherein said blood interface zonedefines pores having a diameter and depth of from 5 to 100 microns. 4.An arterial graft prosthesis as defined in claim 2 wherein the surfaceof said blood interface zone is hydrophilic.
 5. An arterial graftprosthesis as defined in claim 4 wherein the surface of said bloodinterface zone is coated with an antithrombin.